Most commercial low density polyethylenes are polymerized in heavy walled autoclaves or tubular reactors at pressures as high as 50,000 psi and temperatures up to 300.degree. C. The molecular structure of high pressure low density polyethylene is highly complex. The permutations in the arrangement of its simple building blocks are essentially infinite. High pressure resins are characterized by an intricate long chain branched molecular architecture. These long chain branches have a dramatic effect on the melt rheology of the resins. High pressure low density polyethylene resins also possess a spectrum of short chain branches generally 1 to 6 carbon atoms in length which control resin crystallinity (density). The frequency distribution of these short chain branches is such that, on the average, most chains possess the same average number of branches. The short chain branching distribution characterizing high pressure low density polyethylene can be considered narrow.
Low density polyethylene can exhibit a multitude of good properties. It is flexible and has a good balance of mechanical properties such as tensile strength, impact resistance, burst strength, and tear strength. In addition, it retains its strength down to relatively low temperatures. Certain of these resins do not embrittle at temperatures as low as -70.degree. C. Low density polyethylene has good chemical resistance, and it is relatively inert to acids, alkalis, and inorganic solutions. It is, however, sensitive to hydrocarbons, halogenated hydrocarbons, and to oils and greases. Low density polyethylene has excellent dielectric strength.
More than 50% of all low density polyethylene is processed into film. This film is primarily utilized in packaging applications such as for meat, produce, frozen food, ice bags, boilable pouches, textile and paper products, rack merchandise, industrial liners, shipping sacks, pallet stretch and shrink wrap. Large quantities of wide heavy gage film are used in construction and agriculture.
Most low density polyethylene film is produced by the tubular blown film extrusion process. Blown film products range from tubes of film which are about two (2) inches in diameter or smaller and which are used as sleeves or pouches, to huge bubbles that provide a lay flat of about twenty (20) feet in width when slit along an edge and opened up, will measure forty (40) feet wide.
Polyethylene can also be produced at low to medium pressures by homopolymerizing ethylene or copolymerizing ethylene with various alpha-olefins using heterogeneous catalysts based on transition metal compounds of variable valence. These resins generally possess little, if any, long chain branching and the only branching to speak of is short chain branching. Branch length is controlled by comonomer type. Branch frequency is controlled by the concentration of comonomer(s) used during copolymerization. Branch frequency distribution is influenced by the nature of the transition metal catalyst used during the copolymerization process. The short chain branching distribution characterizing transition metal catalyzed low density polyethylene can be very broad.
U.S. patent application Ser. No. 892,325 filed Mar. 21, 1978, abandoned, and refiled on Feb. 27, 1979 as Ser. No. 014,414, now U.S. Pat. No. 4,302,566 in the names of F. J. Karol et al and entitled Preparation of Ethylene Copolymers In Fluid Bed Reactor, and which corresponds to European Patent Application No. 79100953.3 which was published as Publication No. 004,645 on Oct. 17, 1979, discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratio of .gtoreq.22 to .ltoreq.32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities if the monomer(s) are polymerized in a low pressure gas phase process with a specific high activity Mg-Ti containing complex catalyst which is blended with an inert carrier material.
U.S. Patent Application Ser. No. 892,322 filed Mar. 31, 1978, now abandoned and refiled on Feb. 16, 1979 as Ser. No. 012,720, now U.S. Pat. No. 4,302,565 in the names of G. L. Goeke et al and entitled Impregnated Polymerization Catalyst, Process For Preparing, and Use For Ethylene Copolymerization, and which corresponds to European Patent Application No. 79100958.2 which was published as Publication No. 004,647 on Oct. 17, 1979, discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow of .gtoreq.22 to .ltoreq.32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities if the monomer(s) are polymerized in a low pressure gas phase process with a specific high activity Mg-Ti containing complex catalyst which is impregnated in a porous inert carrier material.
U.S. patent application Ser. No. 892,037 filed Mar. 31, 1978, abandoned, and refiled on Feb. 27, 1979 as Ser. No. 014,412, in the names of B. E. Wagner et al and entitled Polymerization Catalyst, Process for Preparing And Use For Ethylene Homopolymerization, and which corresponds to European Patent Application No. 79100957.4 which was published as Publication No. 004,646 on Oct. 17, 1979, discloses that ethylene homopolymers having a density of about .gtoreq.0.958 to .ltoreq.0.972 and a melt flow ratio of about .gtoreq.22 to about .ltoreq.32 which have a relatively low residual catalyst residue can be produced at relatively high productivities for commercial purposes by a low pressure gas phase process if the ethylene is homopolymerized in the presence of a high activity Mg-Ti containing complex catalyst which is blended with an inert carrier material. The granular polymers thus produced are useful for a variety of end-use applications.
The polymers produced in such United States patent applications, hereinafter called The Prior U.S. Applications, when used in an extruder, melt rapidly and experience slow shear thinning.
The polymers as produced by the process of said applications using the Mg-Ti containing complex catalyst possess a narrow molecular weight distribution, Mw/Mn, of about .gtoreq.2.7 to .ltoreq.3.6, and preferably, of about .gtoreq.2.8 to .ltoreq.3.4.
The rheology of polymeric material depends to a large extent on molecular weight and molecular weight distribution.
In film extrusion, two aspects of rheological behavior are important: shear and extension. Within a film extruder and extrusion die, a polymeric melt undergoes severe shearing deformation. As the extrusion screw pumps the melt to and through the film die, the melt experiences a wide range of shear rates. Most film extrusion processes are thought to expose the melt to shear at rates in the 100-5000 sec.sup.-1 range. Polymeric melts are known to exhibit what is commonly termed shear thinning behavior, i.e., non-Newtonian flow behavior. As shear rate is increased, viscosity (the ratio of shear stress, .tau., to shear rate, .gamma.) decreases. The degree of viscosity decrease depends upon the molecular weight, its distribution, and molecular conformation, i.e., long chain branching of the polymeric material. Short chain branching has little effect on shear viscosity. In general high pressure low density polyethylenes have a broad molecular weight distribution and show enhanced shear thinning behavior in the shear rate range common to film extrusion. Narrow molecular weight distribution resins of the present invention exhibit reduced shear thinning behavior at extrusion grade shear rates. The consequences of these differences are that the present narrow distribution resins require higher power and develop higher pressures during extrusion than the high pressure low density polyethylene resins of broad molecular weight distribution and of equilvalent average molecular weight.
The rheology of polymeric materials is customarily studied in shear deformation. In pure shear the velocity gradient of the deforming resin is perpendicular to the flow direction. The mode of deformation is experimentally convenient but does not convey the essential information for understanding material response in film fabrication processes. As one can define a shear viscosity in terms of shear stress and shear rate, i.e.: EQU .eta. shear=.tau.12/.gamma.
where
.eta. shear=shear viscosity (poise) PA1 .tau. 12=shear stress (dynes/cm.sup.2) PA1 .gamma.=shear rate (sec.sup.-1) PA1 .eta. ext=.sigma..sub..parallel. /.epsilon. PA1 .eta. ext=extensional viscosity (poise) PA1 .sigma..sub..parallel. =normal stress (dynes/cm.sup.2) PA1 .epsilon.=strain rate (sec.sup.-1) PA1 L(t)=jaw separation at time t. PA1 L.sub.o =initial jaw separation PA1 .epsilon.=strain rate (sec.sup.-1), a constant PA1 t=time
an extensional viscosity can be defined in terms of normal stress and strain rate, i.e.,:
In pure extensional flow, unlike shear flow, the velocity gradient is parallel to the flow direction. Commercial extrusion processes involve both shear and extensional deformations. In film extrusion (tubular blow and slot cast) the extensional rheology characteristics of a resin are exceedingly important. They may, in fact, dominate the process.
Extensional viscosity can be measured by a number of experimental techniques (see, for example, J. L. White, Report No. 104 of the Polymer Science and Engineering Dept., Univ. of Tenn., Knoxville). The procedure used herein is a constant strain rate method. Briefly, the method uses a servo-controlled Instron tensile testing machine. The ends of a molten ring of polymer, immersed in a silicone oil bath, are separated at an accelerating rate according to the following relationship: EQU L(t)=L.sub.o exp (.epsilon.t) (3)
where
A force transducer measured load during the deformation. Extensional velocity is calculated by dividing stress by strain rate and is determined as a function of displacement or time during the deformation (Temp..about.150.degree. C.).
When high pressure low density polyethylene melts are deformed according to equation (3), extensional viscosity is observed to increase at an accelerating rate with log time. This behavior is shown in FIG. 1 for high pressure polymerized low density polyethylene having a melt index of 0.65 and a density of 0.922. The melt is said to strain harden. This strain hardening intensifies as the strain rate is increased. In some cases the melt may exhibit unbounded stress growth.
The ethylene polymers, as described in The Prior U.S. Applications, do not, in general, show unbounded stress growth. Certain broad molecular weight distribution resins do strain harden, but their extensional viscosity seems to increase linearly with log time (See FIG. 2). The narrow molecular weight distribution resins as described in The Prior U.S. Applications, for example, show little strain hardening when strain rates are low. FIG. 3 shows that strain hardening intensifies at higher strain rates but not to the degree observed in high pressure low density polyethylene or ethylene hydrocarbon copolymers having broad molecular weight distribution.
High pressure low density polyethylene can be considered "soft" in shear and "stiff" in extension when compared to ethylene hydrocarbon copolymers of narrow molecular weight distribution. Ethylene hydrocarbon copolymers having a narrow molecular weight distribution exhibit the opposite rheology. They are "stiff" in shear and "soft" in extension. The terms "soft" and "stiff", as used herein, refer to the relative magnitude of shear and extensional viscosity when comparing the rheology of high pressure low density polyethylene and narrow molecular distribution polymers of the present invention.
High pressure low density polyethylene is generally formed into film by melt extruding the polyethylene, in an extruder having an extrusion screw with a length to diameter ratio of greater than 21:1. In a commercial operation for forming film from high pressure low density polyethylene, the extrusion screw has a length to diameter ratio of 24:1 or longer. These long extrusion screws use mixing device to get maximum output rates and acceptable film quality (melt temperature homogeneity). If an extrusion screw, with length to diameter ratios between 15:1 to 21:1 is used to melt extrude high pressure low density polyethylene, film quality is unacceptable due to poor temperature uniformity. The quality of the film can be improved using an extrusion screw with length to diameter ratio between 15:1 to 21:1 if the extrusion screw is cooled. However, cooling the extrusion screw is not commercially feasible since it drastically reduces output rates and increases power consumption per pound. This can be summarized in the following Table (for blown film extrusion):
TABLE ______________________________________ Maxi- mum Power Extru- Con- Cool- Type of der Power sump- ing Low Extrusion Rate at Con- tion Limit- Density Screw Film.sup.(a) Full sump- Per ed Poly- (L/D) Quality RPM tion Pound Rate ethylene ______________________________________ 20:1 or + 1.0 1.0 1.00 1.0 High &gt;20:1 Pressure 15:1-21:1 - 0.80 0.70 0.88 1.1 High Pressure 15:1-21:1 + 0.50 0.80 1.60 1.0 High (Cooled) Pressure 15:1-21:1.sup.(b) + 0.95 1.0 1.00 1.0 Low Pressure ______________________________________ .sup.(a) film quality with (-) designation lacks temperature uniformity. .sup.(b) For the low density resins described herein, an extrusion screw of about 15:1-21:1 will provide satisfactory melt quality as defined by pressure variations at the head of the extruder.
The unique rheological behavior of the narrow molecular weight polyethylene resins produced as described in The Prior U.S. Applications, for example, manifest itself in several ways in forming film in the tubular blown film extrusion process. If these resins are processed on commercially available equipment, i.e., extruders wherein the extrusion screw has a length to diameter ratio &gt;21:1 particularly 24:1, power consumption is high and melt temperatures are high causing either extruder power or bubble cooling to limit the output rate below that experienced with high pressure low density polyethylene resins.
The use however, of an extrusion screw having a length to diameter ratio of between 15:1-21:1 in an extruder used to melt extrude these narrow molecular weight ethylene polymers into film reduces power consumption and melt temperature and produces good film quality.
Films suitable for packaging applications must possess a balance of key properties in order to meet the performance requirements essential for broad end use utility and wide commercial acceptance. These properties include film optical quality, for example, haze, gloss, and see-through characteristics. Mechanical strength properties such as puncture resistance, tensile strength, impact strength, stiffness, and tear resistance are important. Vapor transmission and gas permeability characteristics are important considerations in perishable goods packaging. Performance in film converting and packaging equipment is influenced by film properties such as coefficient of friction, blocking, heat sealability, and flex resistance. High pressure low density polyethylene has a wide range of utility such as in food packaging and non-food packaging applications. Bags, commonly produced from low density polyethylene, include shipping sacks, textile bags, laundry and dry cleaning bags and trash bags. Low density polyethylene film can be used as drum liners for a number of liquid and solid chemicals and as protective wrap inside wooden crates. Low density polyethylene film can be used in a variety of agricultural and horticultural applications such as protecting plants and crops, as mulching, for storing of fruits and vegetables. Additionally, low density polyethylene film can be used in building applications such as a moisture or moisture vapor barrier. Further, low density polyethylene film can be coated and printed for use in newspapers, books, etc.
Possessing a unique combination of the aforedescribed properties, high pressure low density polyethylene is the most important of the thermoplastic packaging films. It accounts for about 50% of the total usage of such films in packaging. Films made from the polymers of the present invention preferably the ethylene hydrocarbon copolymers, offer an improved combination of end use properties and are especially suited for many of the applications already served by high pressure low density polyethylene.
An improvement in any one of the properties of a film or an improvement in the extrusion characteristics of the resin or an improvement in the film extrusion process itself is of the utmost importance regarding the acceptance of the film as a substitute for high pressure low density polyethylene in many end use applications.